Last Days of a Great City?

For 17 years, the Zoo's leaf-cutter ant queen has laid countless eggs. But now she's slowing down. What does that mean for her vast family?

By Lindsay Renick Mayer

When you’re lucky enough to lay your eyes on the queen ant in the leaf-cutter ant colony at the Smithsonian’s National Zoo, there’s no doubt that she’s the one responsible for the entire complex operation. First there is her astounding size. She’s more than an inch in length, at least ten times larger than her smallest workers. But there’s also an air about her, the result of how the smaller ants, all daughters, treat her. Ever attentive, they appear to care for her tenderly.

Leaf-cutter ants get their name from their habit of cutting out pieces of leaves and bringing them back to the nest. (Mehgan Murphy/NZP)

Perhaps they know what we at the Zoo fear these days—that without the queen, the colony of Atta cephalotes, which stood at a million ants in its heyday, would cease to exist. The queen is 17 now, elderly, and produces significantly fewer eggs than before. Her end may be near.

Because her offspring have a much, much shorter lifespan—between 60 and 90 days—the colony will swiftly die off once the queen dies and stops laying eggs. And whether she has mere months or years left, invertebrate keepers are preparing for the challenges and opportunities that will come with starting a colony afresh.

“I’m really grateful the queen has made it this long,” says animal keeper and entomologist Donna Stockton, who has worked with this colony for eight years. “I’ll be sad when she dies, but the colony has provided so much wonderment for so many people over the years, including me. These ants are just amazing.”

Ant Agriculture

A leaf-cutter ant colony starts with a single female, a queen. Yet at its peak, the colony may consist of millions of ants that each take on one of more than 22 different jobs to keep the colony running seamlessly. There are foragers, soldiers, nurses, gardeners, and even trash collectors—all female.

All those working women start out as the same sort of egg. Yet the care each young ant receives in the nursery determines the job she will be charged with for her entire life. That job will set even the ant’s basic morphology. A soldier will be large, with huge mandibles; a gardener will be tiny to fit in crevices.

Scientists are still unsure how the colony knows which types of workers are needed to keep things running. Researchers do know that the ants send out pheromones and other chemical messages to “talk.”

“We understand the pheromones and how those are sent out, but we think there’s probably more communication going on than we originally thought,” Stockton says. “Perhaps there’s a reporter ant who goes out and does a census and then tells the queen what types of workers the colony needs. But somehow the queen always knows what’s going on in every nook of the colony and is responsive to that.”

Leaf-cutter ants and humans are the only creatures known to grow their own food. (Mehgan Murphy/NZP)

Though its members work different jobs, the whole colony hums to a single goal: sustenance. That sustenance takes the form of a fungus found only in leaf-cutter ant colonies. Growing the fungus is the ants’ livelihood. Leaf-cutter ants are the only animals, other than humans, that farm their own fresh vegetation extensively.

The process starts with small foraging ants. They carve highways the length of a few soccer fields through the floor of the rainforest. They even climb trees, creeping up into the canopy. Along the way, and under the protection of the larger soldiers, they cut bits of flowers and leaves and bring them back to the colony. An ant’s load is weighty. It’s been compared to a human’s carrying 500 pounds.

When the foragers come back with their selections, a group of smaller ants, called hoarders, analyze the findings to determine which are suitable. The hoarders reject waxy samples and anything with anti-fungal properties. Next they take the bits of usable vegetation to the smallest ants, the gardeners. They work in a chamber about the size of a football.

The gardeners take the selected pieces and chew them up, breaking them down with an enzyme. The vegetation turns into pulp. The ants then pluck fungal hyphae, threads smaller than the diameter of a human hair. The gardeners plant the hyphae in the newly deposited leaf pulp. The hyphae feed on the pulp, growing into tasty fungus.

The ants tend their garden meticulously, pulling weeds and ensuring that no harmful pathogens destroy the crop. “Really it’s no different from you tending your own garden at home,” says Ed Smith, a biologist who worked with the Zoo’s earliest leafcutter ant colonies. “Successful gardening rests on successful tending. You have to obligate your time to fussing and weeding. The weeding has to be done with tools of the appropriate size, and these ants have the perfect mandibles to do so.”

And then dinner—the bulb-shaped part of the fungus—is served. Smaller ants feed the soldiers, whose mandibles are too big for them to be able to comfortably mash up food on their own. The queen, too, is brought her meals, though she can move around the colony on her own. In part, researchers believe, the queen determines the health of her colony based on the quality of the fungus she is fed. Finally, the trash ants haul the waste to the garbage chambers, away from the rest of the colony.

Colony Creation

The fungus is also essential to the start of a new colony. Every so often, queen ants in the wild will lay eggs that grow up into queens and drones. They are the only leaf-cutter ants with wings. Seasonally, all of the new queens and drones, which are usually between half an inch to an inch in length, leave their colony and fly into the air to mate. Each queen mates with about half a dozen drones to mix genetic material broadly before returning to the ground. There she sheds her now useless wings and digs into the ground to start her own colony. She will carry that day’s sperm with her to produce eggs for the rest of her life. Yes, even for 17 years.

In addition to having a special sac to store the sperm, she has stored another essential ingredient for starting a new colony—a bit of fungus given to her by her native colony. “It’s like friendship bread,” Smith says. “But this is more than friendship; this is essential.” The queen seals herself underground with that fungus and begins to lay eggs. The fungus is so vital that, instead of feeding the scant resource to her first batch of daughters, the queen gives them eggs to eat. Later, when more workers hatch, they will break through the seal and begin foraging for leaves to feed the fungus, marking the birth of a new colony.

Again and again, that scene plays out in the Western Hemisphere. Leaf-cutter ants live in both North and South America. Their diversity—41 different species—is most notable in the tropics, where they till nearly 45 percent of the land. Leafcutter ants contribute to rainforest soil by composting plant material. Their pruning also keeps vegetation in check.

Leaf-cutter ants can heft leaf pieces larger than their own bodies. (Mehgan Murphy/NZP)

Unlike many other animals at the National Zoo, leaf-cutter ants are not in danger of going extinct anytime soon. Instead, they appear to be resilient. When rainforest areas are clear-cut for farmland, farmers swiftly realize that not many crops will deter leaf-cutter ants. The ants may disappear briefly due to pesticides or forage changes, but they soon return.

Yet leaf-cutter ants are not invincible. For various reasons, foragers face a 35 percent mortality rate every time they go out into the field. One of their most prevalent predators is the phorid fly. It lands on an ant’s back and lays eggs, which hatch and eat the ant from the inside out. To deflect predators, ants climb on other ants’ backs while holding leaves in their mouths. Researchers are not sure if this is an attempt to distract the enemy or to appear fiercer.

Looking Ahead

The Smithsonian is committed to displaying these incredible insects. The Zoo has had a leaf-cutter ant colony since the Invertebrate Exhibit opened in 1987. Maintaining the colony has been a matter of trial and error. Keepers once tried to make the queen more visible by creating a separate chamber for her. This did not sit well with the ants. Determined to move the queen, they accidentally decapitated her by pulling her through a small hole.

Today, the ants inhabit a structure with multiple chambers and extensive overhead tubes. And the Invertebrates team hopes to upgrade the exhibit after the queen dies. For instance, Stockton would like the new colony to include a monitor that allows visitors to see the ants up close. She’d also love to have an open area where the ants can roam. Ants will not cross oil, so this would be a board surrounded by a moat of oil.

A new queen will cost $1,000. The Zoo will purchase her from a colony at a university. That may seem like a high price, but the queen will be invaluable in introducing visitors to invertebrates, Stockton says. They’re fascinating animals, she argues, but they don’t always get the kind of love that the larger, cuddly mammals do.

“Many people think that ants are like little robots that are programmed to do a certain thing,” Stockton says. But visitors to the Zoo’s leaf-cutter ant colony soon learn otherwise. They discover that the ants can communicate through chemical and physical messages, such as the drumming of other ants’ abdomens on leaves.

“When visitors learn,” Stockton continues, “that these tiny animals work with each other in such a way as to create this amazing colony of farmed fungus, it is hard for them to just walk away from the exhibit.”

—Lindsay Renick Mayer is a public affairs specialist for the Smithsonian’s National Zoo.